PROJECT SUMMARY gp96 is an immune chaperone in the lumen of the endoplasmic reticulum, for the folding and assembly of multiple key receptors in immune responses including TLRs and integrins. Dysregulation of gp96 has been linked with a number of rheumatoid diseases such as lupus. We discovered very recently that gp96 is also critically required for the stability and in vivo suppressive function of regulatory T cells (Zhang et al., J Clin Invest 2015). Furthermore, we unveiled that gp96 serves as an essential chaperone for folding the cell surface docking receptor for TGFb, GARP (known also as LRRC32), whose expression is thought to be restricted to Tregs and platelets. Intriguingly, we found that B cells but not myeloid cells express GARP upon TLR ligation, and there is a unique population of GARP+ B cells in the steady state in both mouse lupus models and human patients with SLE. In essence, we have now established that gp96 regulates several key aspects of immunity and tolerance. It is required for optimal dendritic cell activation via chaperoning TLRs to initiate immune responses. gp96 also plays essential roles for immune tolerance by endowing suppressive function of Treg cells. The immunity vs. tolerance fate determination thus could be orchestrated by gp96 in a contextual and cell type specific fashion. We hypothesize that gp96 controls immune tolerance by regulating the bioavailability of cell surface TGFb for Tregs and B cells. This is accomplished by the intrinsic chaperone function of gp96 in folding both GARP and integrins, the former is involved in snatching and binding latent TGFb from the extracellular milieu in a paracrine fashion, whereas the latter is involved in TGFb activation. This novel hypothesis will be addressed by the following two specific aims: Our first Aim will focus on understanding the mechanisms of gp96 in controlling Treg cell function. Our second Aim will determine the roles of B cell-intrinsic GARP in fundamental B cell biology and the pathogenesis of lupus, by taking advantage of our exciting preliminary data demonstrating the roles of GARP in attenuating experimental lupus. Thus, the two aims will be pursued in both normal and pathogenic conditions. A number of novel genetic models including inducible and cell-specific GARP knockout mice, will be used. We believe that full elucidation of our hypothesis will have fundamental implications in understanding the elusive roles of cell surface TGFb in immune tolerance, as well as in the pathogenesis of lupus.